1. Field of the Invention:
This invention relates in general to offshore drilling equipment for oil and gas, and in particular to a system for tensioning a section of casing extending between a subsea wellhead at the sea floor and a surface wellhead located on a platform at the surface.
2. Description of the Prior Art:
When offshore drilling with a jackup drilling rig, the operator will first drill a large diameter bore and install a section of conductor. Various strings of casing will then be installed as the well is drilled deeper. A typical technique when installing the casing is to utilize mudline hangers. With a mudline hanger, the annulus of the casing is not sealed at the subsea wellhead, rather the casing extends through the subsea wellhead to a surface wellhead where an annulus seal seals around each section of casing. The surface wellhead or casing head is located on the platform normally about 90 feet below the drilling rig floor. The surface wellhead is supported by the conductor pipe extending from the subsea wellhead. At the subsea wellhead, a mudline latch on the casing engages a mudline hanger which transfers the weight of the casing it into the subsea wellhead housing.
When installing a string of casing, the operator will lower the string of casing into the well and latch it to the mudline hanger. The operator then pumps cement down the casing which flows back up the annulus surrounding the casing. Then the operator will open wash ports at the subsea wellhead and pump water down the casing to clean the area around the mudline hanger of cement. After the cement has set, the operator will apply tension to the portion of the casing that extends between the subsea wellhead assembly and the surface wellhead assembly, and secure the upper end of the casing in tension.
Various systems are used for adjusting the length of the casing between the subsea wellhead and surface wellhead and applying tension. These systems employ a variety of adjustable load shoulders, including rings mounted to the casing which operate with rotation, rings that mount to the casing and adjust by ratcheting, and others that move casing head shoulders.